Transistor providing different threshold voltages and method of fabrication thereof

ABSTRACT

A transistor includes a channel region with a first portion and a second portion. A length of the first portion is smaller than a length of the second portion. The first portion has a higher threshold voltage than the second portion. The lower threshold voltage of the second portion allows for an increased ON current. Despite the increase attained in the ON current, the higher threshold voltage of the first portion maintains or lowers a relatively low OFF current for the transistor.

TECHNICAL FIELD

The present invention relates in general to semiconductor devices and fabrication and more particularly to a transistor providing different threshold voltages and method of fabrication thereof.

BACKGROUND

The threshold voltage of a transistor has an effect on the ON current and the OFF leakage current for the transistor. Typically for low voltage applications, it is desirable to have a relatively high ON current. The ON current is affected in simplified terms by a scale factor of (V_(gs)−V_(th))^(m) where V_(gs) is the gate to source voltage, V_(th) is the threshold voltage, and m is an exponent between 1 and 2. V_(gs) typically can be the supply voltage. The ON current can be increased by lowering the threshold voltage. However, it is also desirable to have a relatively low OFF leakage current. Lowering the threshold voltage causes an increase in the OFF leakage current. The OFF leakage current is affected in simplified terms by a scale factor of 10^((Vgs−Vth)/S) where S is the subthreshold slope V_(th)×ln 10. In this case, V_(gs) typically can be the voltage of the source. Thus, a change in the threshold voltage causes a desirable change in scale for the ON current but an undesirable change in scale for the OFF leakage current of the transistor and vice versa.

SUMMARY

From the foregoing, it may be appreciated by those skilled in the art that a need has arisen for maintaining or increasing the ON current of a transistor without also increasing the OFF current of the transistor. In accordance with the present invention, a transistor providing different threshold voltages and method of fabrication thereof are provided that substantially eliminate or greatly reduce disadvantages and problems found in conventional transistor designs.

According to an embodiment of the present invention, there is provided a transistor that includes a channel region with a first portion and a second portion. A length of the second portion can be smaller than a length of the first portion. The second portion has a higher threshold voltage than the first portion. The lower threshold voltage of the first portion allows for an increased ON current. Despite the increase attained in the ON current, the higher threshold voltage of the second portion maintains a relatively low OFF current for the transistor.

The present invention provides various technical advantages over conventional transistor designs. Some of these technical advantages are shown and described in the description of the present invention. Certain embodiments of the present invention may enjoy some, all, or none of these advantages. Other technical advantages may be readily apparent to one skilled in the art from the following figures, description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and the advantages thereof, reference is made to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals represent like parts, in which:

FIGS. 1A-1C illustrate a process for fabricating an embodiment of a channel region of a transistor;

FIGS. 2A-2C illustrate an alternative process for fabricating an embodiment of a channel region of the transistor;

FIG. 3 shows a schematic circuit diagram for the transistor;

FIG. 4 shows an alternative embodiment for the channel region;

FIG. 5 shows an alternative embodiment for the channel region.

DETAILED DESCRIPTION

FIGS. 1A-1C show a process for fabricating an embodiment of a channel region 100 of a transistor 10. In FIG. 1A, channel region 100 is implanted with a dopant selected to provide a relatively low threshold voltage for transistor 10. In FIG. 1B, a photoresist mask 102 is formed over channel region 100 to establish a first portion 120 and leave a second portion 122 exposed. The second portion 122 of channel region 100 is implanted with a dopant selected to provide a relatively high threshold voltage. In FIG. 1C, a finished channel region 100 is shown with first portion 120 and second portion 122. The remainder of transistor 10 to include a gate region 104, a source region 106, and a drain region 108 can be formed in a conventional manner.

FIGS. 2A-2C show an alternative process for fabricating an embodiment of channel region 100 of transistor 10. In FIG. 2A, channel region 100 is implanted with a first dopant selected to provide a relatively low threshold voltage for transistor 10. In FIG. 2B, a gate region 104 is formed over channel region 100. A photoresist mask 102 is formed over channel region 100 and gate region 104 to establish a first portion 120 and leave a second portion 122 exposed. The second portion 122 of channel region 100 is implanted with a second dopant selected to provide a relatively high threshold voltage. The implanting of the second dopant may be performed at an angle relative to transistor 10 to assist in establishing second portion 122 and first portion 120. In FIG. 2C, a finished channel region 100 is shown with first portion 120 and second portion 122. The remainder of transistor 10 to include gate region 104, source region 106, and drain region 106 can be formed in a conventional manner.

In one embodiment, second portion 122 is shown in closer proximity to source region 106 of transistor 10 than first portion 120. Though shown in this manner, transistor 10 can also be fabricated with second portion 122 being in closer proximity to drain region 108 than first portion 120.

FIG. 3 shows a schematic circuit diagram of transistor 10. By establishing first portion 120 and second portion 122 within channel region 100, transistor 10 logically becomes a dual transistor device with a lower threshold voltage transistor 112 serially connected with a higher threshold voltage transistor 114. The total length of channel region 100 is similar to channel lengths of conventional transistors. Higher threshold voltage transistor 114 has a channel length of α×L, where L is the total channel length of channel region 100 and α is a fraction less than one. Lower threshold voltage transistor 112 has a channel length of L−(α×L). By optimizing ct to be a small fraction (such as approximately 0.1 to 0.15), the ON current can mainly be determined by lower threshold voltage transistor 112 having a longer channel length and the OFF current can mainly be determined by higher threshold voltage transistor 114 having the shorter channel length.

Higher threshold voltage transistor 114 reduces leakage current exponentially as the threshold voltage is increased. However, this has a minimal affect on the ON current of transistor 10 because the ON current is a quadrature or linear function of threshold voltage as well as from higher threshold transistor 114 having a smaller channel length. The increase resistance provided by higher threshold transistor 114 enables the threshold voltage of lower threshold transistor 112 to become even lower and still maintain a lower OFF current than a traditional transistor design with a uniform channel region. Thereby, the ON current can be further increased. In addition, the OFF leakage current from lower threshold voltage transistor 112 is essentially blocked by higher threshold voltage transistor 114. In essence, higher threshold voltage transistor 114 acts somewhat as an insulator in the OFF state to absorb the leakage current flowing through lower threshold voltage transistor 112. If maintaining low leakage current is of primary interest, then α can be larger (e.g., greater than 0.5).

FIG. 4 shows an alternate embodiment of channel region 100 of transistor 10. Though shown in FIG. 3 as having higher threshold voltage transistor 114 in series with lower threshold voltage transistor 112, channel region 100 may be implanted in a manner to provide any number of transistors within physical transistor 10. For example, channel region 100 may have a first portion 220, a second portion 222, and a third portion 224. Second portion 222 may establish a lower threshold voltage transistor 212. First portion 220 and third portion 224 may establish higher threshold transistors 214 and 216. First portion 220 and third portion 224 may be of equal or different lengths. Similarly, second portion 222 may establish a higher threshold voltage transistor 212. First portion 220 and third portion 224 may establish lower threshold transistors 214 and 216. In essence, this is an extension of the configuration discussed above with reference to FIGS. 1A-1C and 2A-2C, where second portion 122 can be formed to separate first portion 120 into two sub-portions with each sub-portion having a similar doping profile or further processed to have different doping profiles. Further, second portion 122 may be positioned such that each sub-portion may have similar or different lengths.

FIG. 5 shows an alternative embodiment of channel region 100 of transistor 10. In this embodiment, second portion 122 has a lesser depth than first portion 120. Second portion 122 may be implanted to provide a relatively higher threshold voltage. Second portion 122 may also be formed to include a relatively same number of dopants as it would have if it were fabricated with the same depth as first portion 120. The amount of dopant in second portion 122 is merely confined to a smaller area, resulting in higher dopant concentration. This provides an advantage when the drain voltage of second portion 122 becomes higher. Without this embodiment, the higher drain voltage will cause more DIBL (Drain Induced Barrier Lowering) and CLM (Channel Length Modulation) effects.

By selecting dopants to establish different threshold voltages in transistor 10, the ON current for transistor 10 can be increased without causing a corresponding increase in the OFF leakage current of transistor 10. In one example embodiment having a 100 nm length for second portion 122 and an 800 nm length for first portion 120 providing a 900 nm length for channel region 100, second portion 122 can provide a 150 mV higher threshold voltage and first portion 120 can provide a 150 mV lower threshold voltage than a single dopant implanted channel region. The OFF leakage current may be reduced by a factor of five with a corresponding 35% increase in ON current. Moreover, this technique can be applicable for any type of transistor, including junction field effect and metal oxide semiconductor field effect transistor designs.

Thus, it is apparent that there has been provided, in accordance with the present invention, a transistor with dual threshold voltages and method of fabrication thereof that satisfies the advantages set forth above. Although the present invention has been described in detail, various changes, substitutions, and alterations may be readily ascertainable by those skilled in the art and may be made herein without departing from the spirit and scope of the present invention as set out in the appended claims. Moreover, the present invention is not intended to be limited in any way by any statement made herein that is not otherwise reflected in the following claims. 

1. A method for fabricating a transistor, comprising: a first portion of a channel region with a first depth; second portion of the channel region with a second depth, wherein the second depth is lesser than the first depth; implanting the first portion of the channel region with a dopant to provide a first threshold voltage for the transistor; protecting the first portion of the channel region leaving the second portion of the channel region exposed; identifying an amount of the dopant at the first depth that has a desired voltage characteristic; and implanting the exposed second portion of the channel region with the identified amount of the dopant to provide a second threshold voltage for the transistor different than the first threshold voltage, wherein the implanting of the second portion is performed at an angle offset relative to a line perpendicular to the channel region.
 2. The method of claim 1, wherein the second threshold voltage is higher than the first threshold voltage.
 3. The method of claim 2, wherein the first threshold voltage causes an increase in an ON current for the transistor while the second threshold voltage causes a decrease in the OFF current for the device.
 4. The method of claim 1, further comprising: forming the first portion with a first length; forming the second portion with a second length, the second length being smaller than the first length.
 5. The method of claim 4, wherein the second length is determined by an equation α×L and the first length is determined by an equation L−(α×L), where L is a length of the channel region and α is a fraction less than one.
 6. The method of claim 1, further comprising: forming the second portion in closer proximity to a source region of the transistor than the first portion.
 7. The method of claim 1, wherein the second portion is implanted to separate the first portion into two sub-portions.
 8. The method of claim 1, further comprising: forming a third portion in the channel region, the third portion providing a different threshold voltage than the first portion.
 9. The method of claim 8, wherein the third portion is formed such that the first portion separates the third portion from the second portion. 